Adherent peritoneal cells contain a minor subpopulation of B cells that share some characteristics with macrophages which are non-phagocytic. These cells have potent anti-tumor effect and may be responsible for much of the anti-tumor activity usually ascribed to macrophages. BIBLIOGRAPHIC REFERENCES: Nathan, CF., Hill, V.M., and Terry, W.D.: Isolation of a subpopulation of adherent peritoneal cells with anti-tumor activity. Nature 260: 146-148, 1976. Nathan, C.F. and Terry, W.D.: Decreased phagocytosis by peritoneal macrophages from BCG-treated mice - Induction of the phagocytic defect in normal macrophages with BCG in vitro. Cell. Immunol. 29: 295-311, 1977.